US4720701AExpiredUtility

System with enhanced signal detection and discrimination with saturable magnetic marker

49
Individually held — no corporate assignee on recordPriority: Jan 2, 1986Filed: Jan 2, 1986Granted: Jan 19, 1988
Est. expiryJan 2, 2006(expired)· nominal 20-yr term from priority
Inventors:George J. Lichtblau
G08B 13/2488G08B 13/2408G08B 29/26
49
PatentIndex Score
15
Cited by
4
References
110
Claims

Abstract

An antipilferage system is provided with an improved signal processing system to prevent false alarms. This signal processing involves singly or in combination the improvements of a slew rate limited amplifier, which prevents impulse noise from ringing or being stretched by subsequent filters; a gain-controlled amplifier ahead of said slew rate limited amplifier, the gain of the gain-controlled amplifier being reduced as the noise and/or signal exceeds a minimum threshold level; a video detector which controls the gain-controlled amplifier and serves to maintain a minimum signal to noise ratio; a digital smoothing circuit which averages several "frames" of the entire signal such that random noise is averaged to zero while any signals which are synchronized with the modulation are added together; signal processing circuitry for the recognition of incoming signal pulse shape as compared to a predetermined pulse shape, the output of which is a trigger signal to an alarm. Moreover, blanking is employed to block any signals from passing through the system during specific portions of the modulation period and when no "tag" signals would normally be present in the system. A dead zone elimination circuit is also disclosed along with a power-up timed inhibit circuit to prevent false alarms when the system is turned on or when severe power line transients are present.

Claims

exact text as granted — not AI-modified
What is claimed is: 
     
       1. Apparatus for detecting the presence of an object within an interrogation zone comprising: a marker adapted to be secured to said object;   means for applying in said interrogation zone an electomagnetic field having a periodic waveform;   means for monitoring said field in the vicinity of said interrogation zone and detecting disturbances to said field due to the interaction of said marker with said field so as to produce a monitor signal including pulses and noise;   means for removing from said monitor signal substantially all frequency components in the vicinity of the frequency of said periodic waveform of said applied field;   means for detecting pulses in the remaining frequency components of said monitor signal;   signal discrimination and noise rejection means receiving said detected pulses for determining is said detected pulses were caused by said marker and, if so, to produce an alarm indication signal;   said signal discrimination and noise rejection means including a slew rate limited amplifier, the maximum rate of change of voltage with respect to time of the output of said amplifier being limited to be less than a predetermined value; and   means operative in response to production of said alarm indication signal for indicating the presence of said marker within said interrogation zone.   
     
     
       2. The apparatus of claim 1 and further including a linear low-pass filter receiving the output signal of said slew rate limited amplifier. 
     
     
       3. The apparatus of claim 1 and further including means for generating a gain control signal, an amplifier, means for passing said pulses through said amplifier, and means for varying the gain of said amplifier in response to said gain control signal. 
     
     
       4. The apparatus of claim 3 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-mean-square (RMS) of the detected pulses and the noise. 
     
     
       5. The apparatus claim 3 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and the noise. 
     
     
       6. The apparatus of claim 3 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       7. The apparatus of claim 3 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds very rapidly to such peak but decays slowly to a previously low level, thereby to provide a fast-attack-slow-decay response. 
     
     
       8. The apparatus of claim 1 and further including a gain-controlled amplifier for amplifying said pulses and means for rapidly switching the gain of said amplifier from full gain to a very low gain. 
     
     
       9. The apparatus of claim 1 and further including a variable gain-controlled amplifier coupled to said slew rate limited amplifier. 
     
     
       10. The apparatus of claim 1 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before further signal processing. 
     
     
       11. The apparatus of claim 10 wherein said said signal discrimination means includes means for synchronizing the timing of the analog to digital conversion with the periodic waveform of the applied field. 
     
     
       12. The apparatus of claim 11 wherein said synchronizing means includes a phase-locked-loop. 
     
     
       13. The apparatus of claim 1 wherein said signal discrimination means includes means for providing a representation of the expected signal produced by the marker, and a memory for storing said representation. 
     
     
       14. The apparatus of claim 13 where said memory is a digital memory. 
     
     
       15. The apparatus of claim 13 wherein said memory includes one or more threshold detectors and minimum pulse width detectors. 
     
     
       16. The apparatus of claim 1 wherein said signal discrimination means includes means for providing a representation of the expected signal produced by the marker, a memory means for storing said representation and further including means for comparing the detected pulses with the representation in said memory to discriminate the expected marker signal from noise. 
     
     
       17. The apparatus of claim 1 wherein the marker is a magnetic marker and the applied field has sufficient magnetic force to cause magnetic changes in said magnetic marker. 
     
     
       18. The apparatus of claim 17 wherein said magnetic marker includes a strip having a length at least ten times the maximum dimensions of the cross-sectional area. 
     
     
       19. The apparatus of claim 17 and further including a linear low-pass filter receiving said pulses from the output of the said slew rate limited amplifier. 
     
     
       20. The apparatus of claim 17 and further including means for generating a gain control signal, and amplifier receiving said pulses, and means for varying the gain of said amplifier in response to said gain control signal. 
     
     
       21. The apparatus of claim 20 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-means-square (RMS) of the detected pulses and the noise. 
     
     
       22. The apparatus of claim 20 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and the noise. 
     
     
       23. The apparatus of claim 20 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       24. The apparatus of claim 20 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds very rapidly to such peak but decays slowly to a previously low level, thereby to provide a fast-attach-slow-decay response. 
     
     
       25. The apparatus of claim 17 and further including a gain-controlled amplifier for amplifying said pulses and means for rapidly switching the gain of said amplifier from full gain to a very low gain. 
     
     
       26. The apparatus of claim 17 and further including a variable gain-controlled amplifier coupled to said slew rate limited amplifier. 
     
     
       27. The apparatus of claim 17 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before further signal processing. 
     
     
       28. The apparatus of claim 27 wherein said signal discrimination means includes means for synchronizing the timing of the analog to digital conversion with the periodic waveform of the applied field. 
     
     
       29. The apparatus of claim 28 wherein said synchronizing means includes a phase-locked-loop. 
     
     
       30. The apparatus of claim 17 wherein said signal discrimination means includes means for providing a representation of the expected signal produced by the marker, a memory, and storing said representation in said memory. 
     
     
       31. The apparatus of claim 30 where said memory is a digital memory. 
     
     
       32. The apparatus of claim 30 wherein said memory includes one or more threshold detectors and minimum pulse width detectors. 
     
     
       33. The apparatus of claim 17 wherein said signal discrimination means includes means for providing a representation of the expected signal produced by the marker, a memory means for storing said representation in said memory; further including means for comparing the detected pulses with the representation in said memory to discriminate the expected marker signal from noise. 
     
     
       34. The apparatus of claim 17 wherein said field is alternating and wherein said detected pulses and noise are passed through a recirculating delay line integrator with the delay time of the delay line equal to the period of the alternating applied magnetic field. 
     
     
       35. The apparatus of claim 34 wherein said delay line includes a charge coupled device (CCD). 
     
     
       36. The apparatus of claim 34 wherein said recirculating delay line is synchronized to the applied magnetic field. 
     
     
       37. The apparatus of claim 36 and further including a phase-locked-loop synchronization of said recirculating delay line to the applied magnetic field. 
     
     
       38. The apparatus of claim 17 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before further signal processing. 
     
     
       39. The apparatus of claim 38 wherein said signal discrimination includes means for synchronizing the timing of the analog to digital conversion with the periodic waveform of the applied magnetic field. 
     
     
       40. The apparatus of claim 39 wherein said synchronizing means includes a phase-locked-loop. 
     
     
       41. The apparatus of claim 17 and further including means for inhibiting said apparatus from indicating the presence of said magnetic marker for a fixed period of time after power is turned on to said apparatus. 
     
     
       42. The apparatus of claim 1 wherein the marker is a resonant circuit and the applied electromagnetic field is at a freqeuncy which is swept through a range including the resonant frequency of the marker. 
     
     
       43. The apparatus of claim 42 and further including a linear low-pass filter receiving said pulses from the output of said slew rate limited amplifier and then through said linear low-pass filter. 
     
     
       44. The apparatus of claim 42 and further including means for generating a gain control signal, an amplifier receiving said pulses, and means for varying the gain of said amplifier in response to said separate gain control signal. 
     
     
       45. The apparatus of claim 44 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-mean-square (RMS) of the detected pulses and the noise. 
     
     
       46. The apparatus of claim 44 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and the noise. 
     
     
       47. The apparatus of claim 44 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       48. The apparatus of claim 44 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds very rapidly to such peak but decays slowly to a previously low level, thereby to provide a fast-attack-slow-decay response. 
     
     
       49. The apparatus of claim 42 and further including a gain-controlled amplifier for amplifying said pulses and means for rapidly switching the gain of said amplifier from full gain to a very low gain. 
     
     
       50. The apparatus of claim 42 and further including a variable gain controlled amplifier coupled to said slew rate limited amplifier. 
     
     
       51. The apparatus of claim 42 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before further signal processing. 
     
     
       52. The apparatus of claim 51 wherein said signal discrimination means includes means for synchronizing the timing of the analog to digital conversion with the modulation of the swept radio frequency field. 
     
     
       53. The apparatus of claim 52 wherein said synchronizing means includes a phase-locked-loop. 
     
     
       54. The apparatus of claim 42 wherein said signal discrimination means includes means for providing a representation of the expected signal produced by the marker, a memory, and storing said representation in said memory. 
     
     
       55. The apparatus of claim 54 where said memory is a digital memory. 
     
     
       56. The apparatus of claim 54 wherein said memory includes one or more threshold detectors and minimum pulse width detectors. 
     
     
       57. The apparatus of claim 42 wherein said signal discrimination means includes means for providing a representation of the expected signal produced by the marker, a memory means for storing said representation in said memory; further including means for comparing the detected pulses with the representation in said memory to discriminate the expected marker signal from noise. 
     
     
       58. Apparatus for detecting the presence of an object within an interrogation zone comprising: a marker adapted to be secured to said object;   means for applying in said interrogation zone an electromagnetic field having a periodic waveform;   means for monitoring said field in the vicinity of said interrogation zone and detecting disturbances to said field due to the interaction of said marker with said field so as to produce a monitor signal including pulses and noise;   means for removing from said monitor signal substantially all frequency components in the vicinity of the frequency of said periodic waveform of said applied field;   means for detecting pulses in the remaining frequency components of said monitor signal;   receiving said pulses for determining if said detected pulses were caused by said marker and, if so, to produce an alarm indication signal;   said signal discrimination and noise rejection means storing in a digital memory a representation of the signal expected when said member is present in said interrogation zone, said representation comprising a predetermined series of alternating polarity pulses which occur in a predetermined time sequence wherein each of said pulses is approximated by a rectangular voltage waveform with a predetermined minimum amplitude and with a pulse width defined as falling between a predefined minimum and maximum value, and comparing the detected pulses from said monitor to the representation in memory to discriminate the expected marker signal from noise; and   means operative in response to the production of said alarm indication signal for indicating the presence of said marker within said interrogation zone.   
     
     
       59. The apparatus of claim 58 and further including means for generating a gain control signal, an amplifier receiving said monitor pulses, and means for varying the gain of said amplifier in response to said gain control signal. 
     
     
       60. The apparatus of claim 59 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-mean-square (RMS) of the detected pulses and noise. 
     
     
       61. The apparatus of claim 59 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and noise. 
     
     
       62. The apparatus of claim 59 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       63. The apparatus of claim 59 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds rapidly to such peaks but decays slowly to a previously low level, thereby to provide a "fast-attach-slow-decay" response. 
     
     
       64. The apparatus of claim 58 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before further signal processing. 
     
     
       65. The apparatus of claim 64 further including means for synchronizing said analog to digital conversion to the periodic waveform of the applied field. 
     
     
       66. The apparatus of claim 65 wherein said means for synchronizing includes a phase-locked loop. 
     
     
       67. The apparatus of claim 58 wherein the marker is a magnetic marker and the applied field has sufficient magnetic force to cause magnetic changes in said magnetic marker. 
     
     
       68. The apparatus of claim 67 and further including means for generating a gain control signal, an amplifier receiving said monitor pulses, and means for varying the gain of said amplifier in response to said gain control signal. 
     
     
       69. The apparatus of claim 68 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-mean-square (RMS) of the detected pulses and noise. 
     
     
       70. The apparatus of claim 68 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and noise. 
     
     
       71. The apparatus of claim 68 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       72. The apparatus of claim 68 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds rapidly to such peaks but decays slowly to a previously low level, thereby to provide a "fast-attack-slow-decay" response. 
     
     
       73. The apparatus of claim 67 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before signal processing. 
     
     
       74. The apparatus of claim 73 wherein said analog to digital conversion is synchronized with the periodic waveform of the applied field. 
     
     
       75. The apparatus of claim 74 wherein said synchronizing means includes a phase-locked loop. 
     
     
       76. The apparatus of claim 58 wherein the marker is a resonant circuit and the applied electromagnetic field is at a frequency which is swept through a range including the resonant frequency of the marker. 
     
     
       77. The apparatus of claim 76 and further including means for generating a gain control signal, an amplifier receiving said monitor pulses, and means for varying the gain of said amplifier in response to said gain control signal. 
     
     
       78. The apparatus of claim 77 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-mean-square (RMS) of the detected pulses and noise. 
     
     
       79. The apparatus of claim 77 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and noise. 
     
     
       80. The apparatus claim 77 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       81. The apparatus of claim 77 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds rapidly to such peaks but decays slowly to a previously low level, thereby to provide a "fast-attack-slow-decay" response. 
     
     
       82. The apparatus of claim 76 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before further signal processing. 
     
     
       83. The apparatus of claim 82 further including means for synchronizing said analog to digital conversion to the modulation of the swept radio frequency field. 
     
     
       84. The apparatus of claim 83 wherein said means for synchronizing includes a phase-locked loop. 
     
     
       85. The apparatus of claim 58 and further including a recirculating delay line integrator wherein said signals from said monitor are passed through said integrator prior to comparison with said digital representation of said monitor signal. 
     
     
       86. The apparatus of claim 85 wherein said delay line integrator is synchronized with the periodic waveform of the applied field. 
     
     
       87. The apparatus of claim 85 wherein said delay line integrator is implemented in a charge-coupled delay line. 
     
     
       88. The apparatus of claim 85 wherein said delay line integrator is implemented in a digital memory in conjunction with a microprocessor. 
     
     
       89. Apparatus for detecting the presence of an object within an interrogation zone comprising: a marker adapted to be secured to said object;   means for applying in said interrogation zone an electromagnetic field having a periodic waveform;   means for monitoring said field in the vicinity of said interrogation zone and detecting disturbances to said field due to the interaction of said marker with said field so as to produce a monitor signal including pulses and noise;   means for removing from said monitor signal substantially all frequency components in the vicinity of the frequency of said periodic waveform of said applied field;   means for detecting pulses in the remaining frequency components of said monitor signals;   signal discrimination and noise rejection means for receiving said detected pulses for determining if said detected pulss were caused by said marker and, if so, to produce an alarm indication signal;   means for generating a gain control signal, an amplifier, means for passing said monitor pulses through said amplifier, and means for varying the gain of said amplifier in response to said gain control signal, wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise adn responds rapidly to such peaks, but decays slowly to a previously low level, providing a fast-attack-slow-decay response; and   means operative in response to production of said alarm indication signal for indicating the presence of said marker within said interrogation zone.   
     
     
       90. The apparatus of claim 89 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before signal processing. 
     
     
       91. The apparatus of claim 90 wherein said analog ot digital conversion is synchronized with the periodic waveform of the applied field. 
     
     
       92. The apparatus of claim 91 wherein said synchronizing means includes a phase-locked loop. 
     
     
       93. The apparatus of claim 89 wherein the marker is a magnetic marker and the applied field has sufficient magnetic force to cause magnetic changes in said magnetic marker. 
     
     
       94. The apparatus of claim 93 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-means-square (RMS) value of the detected pulses and noise. 
     
     
       95. The apparatus of claim 93 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and noise. 
     
     
       96. The apparatus of claim 93 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       97. The apparatus of claim 93 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before signal processing. 
     
     
       98. The apparatus of claim 97 wherein said analog to digital conversion is synchronized with the periodic waveform of the applied field. 
     
     
       99. The apparatus of claim 98 wherein said synchronizing means includes a phase-locked loop. 
     
     
       100. The apparatus of claim 93 wherein said field is alternating and where in said detected pulses and noise are passed through a recirculating delay line integrator with the delay time of the delay line equal to the period of the alternating applied magnetic field. 
     
     
       101. The apparatus of claim 100 wherein said delay line includes a charge coupled device (CCD). 
     
     
       102. The apparatus of claim 100 wherein said recirculating delay line is synchronized to the applied magnetic field. 
     
     
       103. The apparatus of claim 89 wherein the marker is a resonant circuit and the applied electromagnetic field is at a frequency which is swept through a range including the resonant frequency of the marker. 
     
     
       104. The apparatus of claim 103 wherein the gain of said amplifier is controlled by a signal which is proportional to the true root-mean-square (RMS) value of the detected pulses and noise. 
     
     
       105. The apparatus of claim 103 wherein the gain of said amplifier is controlled by a signal which is proportional to the average absolute value of the detected pulses and noise. 
     
     
       106. The apparatus of claim 103 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise. 
     
     
       107. The apparatus of claim 103 wherein the gain of said amplifier is controlled by a signal which is proportional to the peak value of the detected pulses and noise and responds rapidly to such peaks but decays slowly to a previously low level, thereby to provide a "fast-attack-slow-decay" response. 
     
     
       108. The apparatus of claim 103 and further including means for periodically converting said detected pulses and noise from an analog to a digital representation before signal processing. 
     
     
       109. The apparatus of claim 108 further including means for synchronizing said analog to digital conversion to the modulation of the swept radio frequency field. 
     
     
       110. The apparatus of claim 109 wherein said means for synchronizing includes a phase-locked loop.

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